This invention concerns electron guns used in television picture tubes, particularly those having a unipotential-type electrostatic focus lens, and the problem of electrical arcing in such guns.
Electron guns employed in television picture tubes generally comprise two distinct sections. The first is a rear section made up successively of a thermionic cathode for emitting electrons, a control grid, and an accelerating grid for forming a beam "cross-over." The thus-formed beam enters the influence of a second section commonly called a "main focus lens." The main focus lens is comprised of two or more electrodes between which are formed electrostatic fields which serve to focus an image of the cross-over on the phosphor-bearing viewing screen of the picture tube. Also, by virtue of the potentials on the electrodes, the kinetic energy of the beam is increased.
Most electron guns comprise a series of discrete, electrically conductive discs or tubular elements contiguous to each other and aligned on a common axis. In multi-gun assemblies, each gun may comprise a series of electrically discrete electrodes, or, the electrodes of the guns which have functions in common may be physically combined, or "unitized." Each gun electrode normally may receive a voltage of a predetermined potential to establish electrostatic fields between the electrodes for forming and shaping the beam, and focusing the beam cross-over to provide small, symmetrical "spots" on the viewing screen.
A monochrome television system uses a single gun for activation of the phosphor targets on the viewing screen. Color television systems commonly make use of three guns to activate groups of red-light-emitting, green-light-emitting, and blue-light-emitting phosphors deposited in predetermined patterns on the screen. The three guns may be arranged in a trigonal configuration, or, the array may be "in-line"; that is, the guns may lie side-by-side in the same plane, with the emitted beams being coplanar.
With regard to the main focus lens, there are two types that have long been in use. The first is the two-electrode "bipotential" focus lens, which presents to electrons traveling through the main focus lens an axial potential distribution which increases monotonically from an initial low potential on an electrode adjacent to the rear section to a final high potential on an electrode nearest the viewing screen, as shown diagrammatically in FIG. 1. Because of its initial low potential, a gun having a bipotential lens has little tendency to arc between close lying, adjacent electrodes of the rear section and the main focus lens section.
A second type of main focus lens, termed the "unipotential" focus lens, also known as the Einzel, is comprised of three or more electrodes. The term unipotential refers to a lens whose axial potential distribution is substantially saddle-shaped (high-low-high) and in which the potentials on the first and last electrodes are equal. The axial potential distribution of the lens decreases monotonically from an initial relatively high potential on an electrode adjacent to the rear section, to a relatively low potential on an intermediate electrode, then increases, again monotonically to a final relatively high potential on an electrode nearest the viewing screen. The graphical contour of this potential distribution is shown by FIG. 2.
Unipotential type main focus lenses typically have the minimum three electrodes; however, others use a greater number for improved gun performance (see for example U.S. Pat. No. 3,895,253 assigned to the assignee of the present invention).
Because of this distribution of potentials in unipotential type main focus lenses, wherein the potential on an electrode of the main focus lens nearest the rear section is very high, the unipotential gun exhibits a tendency to arc at the interface of the rear section and the main focus lens section. This arcing is attributable to the great difference in potential between the two sections. For example, the electrode of the rear section nearest the main focus lens may have a potential thereon of 1 kilovolt or less, and is commonly displaced a nominal distance of 40 mils from the adjacent electrode of the main focus lens section. The adjacent electrode of the main focus lens may in turn have a potential of 30 kilovolts in present-day, high-brightness tubes. The great difference in potential of nearly 30 kilovolts, coupled with the smallness of the gap, makes the tendency toward arcing all to possible.
A possible solution leading to arc-inhibition would be to increase the inter-electrode gap. However, increasing the gap reduces the prefocusing strength of the electrostatic lens that exists between the two electrodes; also, an increased gap enhances susceptibility of the lens to external aberrating influences.
As is well known in the art, the effect of arcing can be catastrophic in terms of the operability of the television picture display system. Components of the system that can be damaged include power supply circuits; video drivers in color circuits, especially if they comprise transistors and integrated circuits; and the gun itself.
The result has been that, as operating voltages have increased in response to the consumer demand for greater picture brightness, the unipotential focus lens, with its inherent arcing tendencies, has fallen into relative disuse in favor of the bipotential focus lens. This despite the fact that the unipotential lens, and especially the extended-field-type lens, offers performance superior to the bipotential lens, especially with regard to the reduction of spherical aberration.
U.S. Pat. No. 3,863,091 to Hurukawa discloses a unipotential-type main focus lens system having an additional focus electrode disposed nearest the accelerating grid. The stated purpose of the additional electrode, in combination with the other electrodes of the main focus lens, is to form at least two separate electrostatic focusing lens fields, which in turn form a unitary lens having a large aperture. The purpose is said to be the provision of smaller focusing spots and improved focusing characteristics.
In U.S. Pat. No. 3,558,954 to Lilley, an arc-suppression apparatus for a unipotential type gun is disclosed comprising a low-voltage suppressor ring of wire surrounding and spaced outwardly from and disposed substantially in the beam-accelerating gap between the adjacent low-and high-voltage electrodes. The purpose is said to be to shield the external surfaces of the low-voltage electrodes from the high electric fields of the first high voltage electrodes.